Gellan-based hydrogels and microgels: A rheological perspective

Gellan gum-based systems have gained significant attention due to their versatility for multiple applications. In particular, they have shown a great potentiality in the field of cultural heritage, as efficient paper artwork cleaning agents in restoration processes. This efficacy is enhanced when gellan gum is assembled to form stable microgels, by controlling the gelation process under shear. Moreover, the use of methacrylated gellan gum provides additional functionality to the systems, that are also able to remove hydrophobic residues during the cleaning process. However, in order to optimize the manufacturing process, it is fundamental to obtain a thorough understanding of the rheological behaviour of the employed gellan gels in the optimal working conditions for paper cleaning. The present work aims to thoroughly characterize the rheological properties of low-acyl gellan gum, also during hydrogel and microgel formation, assessing the role of temperature (25-80 °C), gellan concentration (0.5-5 % for hydrogels and 0.1-0.5 % for microgels), methacrylation, presence of different cations (Na⁺, Ca²⁺) and salt concentration (0.25-5.0 mM for hydrogels and 100 mM for microgels), on the behaviour of viscosity and viscoelastic moduli. We find the notable result that gellan hydrogels and microgels exhibit a double yielding behaviour in the conditions where they are mostly efficient for art restoration. Furthermore, we identify the optimal rheological conditions of these gels for efficient artwork restoration, opening the possibility to extend their applications to different substrates and in other fields.